The evolution of reproductive isolation

The development of reproductive isolation (speciation) is a key event in evolution, and hence for the origin and maintenance of the tremendous diversity of life on earth. For sexually reproducing organism there is general agreement that reproductive isolation is a necessity for divergence. Nevertheless, the mechanisms involved in the formation of reproductive barriers are poorly understood. For organisms where reproduction and recombination do not necessarily coincide, species concepts) remain unresolved.

For prokaryotes, in particular, taxonomic boundaries are obscure and it is questionable if a species concept in terms of reproductively isolated units is meaningful (cf. Colloquium 3). In order to understand the high levels of homologous recombination among Thermotoga strains across large geographic and phenotypic distances, global population genetic approaches are needed. A large number of Thermotogales isolates will be sampled worldwide and used in multilocus-sequence-typing and-analysis (MLST; closely related strains, and MLSA; more distantly related isolates). We will investigate population structure and how species-like boundaries (if present) may form in the face of high levels of recombination. In the cases of Yersinia and Salmonella we will explore the evolution of distinct taxonomic units within each genus, focusing on pathogenic strains. This work will be carried out using available DNA sequence data and a bioinformatic approach.

Some passerine birds coexist in certain regions as species that are distinct, but not completely reproductively isolated (hybrid zones). Thus the evolution of species boundaries is still in action. When hybrids are unfit, females should avoid mating with males from the other species. Different signals may provide information of varying reliability regarding species identity and mate quality. We will expand on optimality models developed for a one-species setting to investigate how information content of sexual signals and relative abundance of con- and heterospecifics may affect mate search strategies of females in a hybrid zone and test specific predictions from the models using experimental studies. We will further explore the often-neglected alternative hypothesis that intersexual competition may account for unidirectional hybridization.

For pied and collared flycatcher, it has been documented that unfit hybrids can reinforce prezygotic isolation, for instance through sympatric divergence in traits used in mate recognition. Prezygotic isolation includes lack of sexual attraction between members of differentiated populations and often involves sex-specific traits controlled directly or indirectly by genes linked to the sex chromosomes. We suggest that sex chromosome evolution and the particular architecture of sexlinked genes (hemizygosity and reduced rate of recombination) play a crucial role in speciation. We will investigate speciation traits using genome-wide genotyping of various hybrid and backcrossed birds, and by correlating genotypes with phenotypic traits, such as fertility and viability (post-zygotic isolation traits), mate preferences and sexual signals (pre-zygotic isolation traits).

In the European grayling system several viable demes have been established within 25 generations. The population structure may be a result of founder events (non-random colonization), selection for spawning time (variable environments leading to isolation-bytime) or selection against dispersal (isolation-inspace). We aim to identify the ecological factors that facilitate the strong selection. We will then perform in-situ testing for pre-zygotic isolation mechanisms (mate choice experiments, timing of maturation) and post-zygotic isolation mechanisms (early development, selection against hybrids). Reproductive isolation may follow from development of phenotypic differences. This will be studied by estimating field-based variance-covariance matrices for important traits.

Through polyploidization, reproductive isolation (i.e., new species) may arise in a single or few generations. In certain plants and invertebrates the frequency of polyploids is particularly high in harsh environments such as the Arctic, where asexuality is also common. Polyploidy is thought to be important for maintenance of genetic diversity in the absence of recombination and could also enhance expression of genes. By comparative studies of a range of species, including both diploids and polyploids, we will assess whether particular ecological settings favour the establishment of polyploids and test whether polyploids are more diverse and fitter than diploids in the Arctic. Populations from a wide latitudinal range will be screened for ploidy-level, heterozygosity and for correlations between ploidy-level and important life history (e.g., growth rate, size) and related features (e.g., RNA/DNA-ratios). For Arctic plants with both diploid and polyploid cytotypes genetic and phenotypic characterization, manipulative field studies, experiments under controlled conditions and simulation models incorporating varying ecological conditions and data on rates of clonal growth and seed set will be used.